The invention relates to a digital solid-state trip unit for an electrical circuit breaker. A microprocessor-based trip unit of the kind mentioned processes digital signals resulting from a sampling of the analog signals representative of the currents. Replacing the continuous analog signals by discrete digital signals gives rise to error on the measurement of the peak value of these signals, this error being greater the greater the sampling period in comparison with the period of the signal to be measured. The signal period corresponds to that of the mains and the sampling period is limited by the characteristics of the microprocessor.
It is important to increase the accuracy to ensure protective device discrimination, i.e. to eliminate a fault occurring at a point in the distribution system by the circuit breaker fitted immediately upstream from the fault and by this circuit breaker alone. Discrimination is provided by the difference between the non-tripping time and the tripping or breaking time of the circuit breakers. Two circuit breakers fitted in series are discrimating if the tripping time of the downstream breaker is lower than the non-tripping time of the upstream breaker, whatever the value of the fault current. Inaccuracy of these times limits discrimination or requires higher safety margins, resulting in a delay in breaking the fault current.
The object of the present invention is to improve the precision of the sampled signal for a given sampling period and to achieve a very small difference between the tripping and non-tripping times.